The description of a small amount of pelvic fluid found during a gynecologic ultrasound inadvertently causes panic in many people, who mistakenly believe they must have pelvic inflammatory disease. The following is a brief introduction to pelvic fluid. The pelvic fluid is divided into physiological pelvic fluid and pathological pelvic fluid: 1. A small amount of menstrual blood flowing back into the pelvis during the menstrual period can cause pelvic fluid; 2. Follicular fluid accumulates in the pelvis during ovulation in the middle of the menstrual period resulting in pelvic fluid. Most physiological pelvic effusions are asymptomatic and can disappear naturally without treatment. Pathological pelvic fluid: 1. When there is inflammation in the pelvic cavity or abdominal organs, the inflammatory fluid leaks out and collects at the bottom of the pelvic cavity, which will appear as fluid under ultrasound, usually with discomfort such as lower abdominal cramping and lumbago. 2. 3. In the case of ascites caused by ovarian tumor, not only pelvic fluid but also ascites is found under ultrasound. Pathological pelvic effusion needs to be treated in a regular hospital.